Carbon dioxide flooding processes are an important enhanced oil recovery method to recover oil from both sandstone and carbonate reservoirs. Approximately one third of the original oil in place is recovered by primary and secondary recovery processes. However, this typically leaves two-thirds of the oil trapped in reservoirs as residual oil after water flooding. An additional 5-20% of the oil may be recovered by carbon dioxide flooding processes. However, increasing the recovery beyond this has remained difficult because of several challenges. First is the gravity override of the injected carbon dioxide due to density differences between the injected carbon dioxide and resident fluids in the reservoir. The carbon dioxide, being lighter, tends to rise to the top of the reservoir thereby bypassing some of the remaining oil, This results in poor oil recovery in the lower portion of the reservoir. This problem is especially acute in thick formations. The second challenge is viscous fingering that is caused by the lower viscosity of the injected carbon dioxide. Typical dense carbon dioxide viscosity at reservoir conditions is in the range of 0.05-0.1 cP, which is much lower than the viscosity of resident oil and brine. The resulting unfavorable mobility ratio leads to viscous fingering. This causes early carbon dioxide breakthrough, high carbon dioxide utilization factors, poor sweep efficiency, and low overall oil recoveries. The third challenge is reservoir geology and heterogeneities, including high permeability streaks and fractures that can affect the sweep efficiency of a carbon dioxide enhanced oil recovery flooding processes. While water-alternating-gas processes have shown to improve the mobility of carbon dioxide somewhat, water-alternating-gas processes have not completely overcome these challenges.
Increasing the density and viscosity of carbon dioxide can alleviate many of these challenges and lead to substantial higher recovery than conventional carbon dioxide enhanced oil recovery processes. Carbon dioxide density can be increased by blending in heavier compatible materials. However, limited success has been achieved using this approach, partly because the densities of the molecules that have previously been used are not high enough.
Additionally, known methods use surfactants to foam or to create water in carbon dioxide reverse micelles. While creating a foam addresses the challenge of viscosity, it leaves the challenge of density unresolved. Although research results have demonstrated that surfactant-induced carbon dioxide foams are an effective method for mobility control in carbon dioxide foam flooding, the foam's long-term stability during a field application is difficult to maintain.
Moreover, even if a carbon dioxide thickener, whether a polymer or small molecule, is identified, operational constraints may face operators who would try to implement the technology in a pilot-test. Nearly all potential carbon dioxide thickeners are a solid at ambient temperature and a means of introducing a powder into the carbon dioxide stream must be employed, possibly by first dissolving the thickener in an organic solvent in order to form a concentrated, viscous, pumpable solution.
Reverse micelles carry a small amount of water with a significant amount of surfactant due to the nature of micelles. In other words, micelles carry little payload due to their high surface to volume ratios.
A capsule based carbon dioxide system addresses the density challenge by delivering a substantial amount of a dense liquid, such as water or heavy filler. The surface-volume ratio of the capsule is much smaller than that of reverse micelle, hence more payload can be added to the capsule based carbon dioxide system and more density increase can be realized using such a system.
A capsule based carbon dioxide system also addresses the viscosity challenge because of the drag force of carbon dioxide on the capsules, and the inherent viscosity of carbon dioxide-philic molecules on the capsules.